The objectives of this research are to develop in vivo 23Na MRS/MRI techniques to measure compartmental Na+ and apply the developed techniques to study the role of transmembrane Na+ gradients in different physiological conditions. We have accomplished the following during this reporting period: 1.The potential of multiple quantum filtered (MQF) techniques to detect mainly intracellular Na+ (Na+i) has stimulated several researchers to explore the possibility of MQF imaging. However, low sensitivity has been the major obstacle to MQF 23Na imaging becoming an important technique for biological and clinical applications. To circumvent this problem we use weighted signal averaging and coherent echo addition techniques. A combination of these two techniques provides 70-100% increase in signal-to-noise ratio. The weighted signal averaging technique works by summing different number of acquisitions during different phase encoding steps. In coherent echo addition technique, the alternate echoes from a multi-echo data set are reversed and all the echoes are added together. With the help of these techniques, we are now able to collect 3D MQF 23Na images of the whole rat and dog head in 1-2 hour on the 4.7 T magnet. MQF images of rats collected 24 h after a carbon tetrachloride treatment show substantial increase in signal intensity from the liver. Thus MQF 23Na imaging can be done in a time period acceptable for human applications and it is easy to detect abnormal levels of Na+i noninvasively. 2. We have used SQ 23Na MRI to determine the changes in Na+ signal intensity produced by voluntary muscle contractions in humans. Three-dimensional gradient-echo 23Na images of normal untrained volunteers were collected before and after ankle flexion-extension exercise on a 1.5 T magnet. As compared to a 40 mM NaCl standard the signal intensity from the leg muscle was approximately one-third before exercise. The images collected immediately after exercise showed a 30% increase in signal intensity. Use of the MQ filtered technique should allow us to determine if the observed increase in signal intensity is due to an increase in the extracellular volume or an increase in the Na+i concentration.